This invention relates generally to color imaging and the use of plural exposure and development steps for such purposes and more particularly to the use of one of the corona generating devices performing more than one function.
One method of printing in different colors is to uniformly charge a charge retentive surface and then expose the surface to information to be reproduced in one color. This information is rendered visible using marking particles followed by the recharging of the charge retentive surface prior to a second exposure and development. This recharge/expose/and develop (REaD) process may be repeated to subsequently develop images of different colors in superimposed registration on the surface before the full color image is subsequently transferred to a support substrate. The different colors may be developed on the photoreceptor in an image on image development process, or a highlight color image development process (image next-to image). Each different image may be formed by using a single exposure device, e.g. ROS, where each subsequent color image is formed in a subsequent pass of the photoreceptor (multiple pass). Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image, during a single revolution of the photoreceptor (single pass).
Several issues arise that are unique to the REaD image on image process of creating multi-color images in the attempt to provide optimum conditions for the development of subsequent color images onto previously developed color images. For example, during a recharge step, it is important to level the voltages among previously toned and untoned areas of the photoreceptor so that subsequent exposure and development steps are effected across a uniformly charged surface. The greater the difference in voltage between those image areas of the photoreceptor previously subjected to a development and recharge step; and those bare non-developed, untoned areas of the photoreceptor, the larger will be the difference in the development potential between these areas for the subsequent development of image layers thereon.
Another issue that must be addressed with the image on image color image formation process is the residual charge and the resultant voltage drop that exists across the toner layer of a previously developed area of the photoreceptor. Although it may be possible to achieve voltage uniformity by recharging this previously toned layer to the same voltage level as neighboring bare areas, the associated residual toner voltage (V.sub.t) prevents the effective voltage above any previously developed toned areas from being re-exposed and discharged to the same level as neighboring bare photoreceptor areas which have been exposed and discharged to the actual desired voltage levels. Furthermore, the residual voltage associated with previously developed toner images reduces the dielectric and effective development field in the toned areas, thereby hindering the attempt to achieve a desired uniform consistency of the developed mass of subsequent toner images. The problems become increasingly severe as additional color images are subsequently exposed and developed thereon. Color quality is severely threatened by the presence of the toner charge and the resultant voltage drop across the toner layer. The change in voltage due to the toned image can be responsible for color shifts, increased moire, increased color shift sensitivity to image misregistration and motion quality, toner spreading at image edges, and loss in latitude affecting many of the photoreceptor subsystems. Thus, it is ideal to reduce or eliminate the residual toner voltage of any previously developed toned images.
Prior attempts to address one or more of these issues have introduced a variety of secondary problems, each having an adverse effect on the image on image color image formation process. For example, the copending application for patent entitled "Method and Apparatus for Reducing Residual Toner Voltage", Ser. No. 08/347,616, by a common assignee as the present application, discloses a voltage sensitive recharge device used for the recharging steps during a color image formation, whose graph of the output current (I) to the charge retentive surface as a function of the voltage to the charge retentive surface (V) has a high (I/V) slope. The high I/V slope recharge device disclosed having an AC voltage supplied thereto, enables an extended time for neutralization to occur at the top of the toner layers. However, the amount of residual voltage V.sub.t reduction that can be realized is limited in this system.
Another recharging method is described in application for Japanese Patent No. Hei-03-202869, published Sep. 4, 1991 and Application No. Hei 1-340663, Application date Dec. 29, 1989, Publication date Sep. 4, 1991, assigned to Matsushita Denki Sangyo K.K. This reference discloses a color image forming apparatus wherein a first and second charging device are used to recharge a photoconductor carrying a first developed image, before exposure and development of a subsequent image thereon. The potential of the photoconductor is higher after passing the first charging device than after passing the second charging device. This reference teaches that the difference in voltage applied by the first and second charging devices to the toner image and photoreceptor surface is set to a relatively high level, to insure that the polarity of the toner image is reversed after passing and having been charged by both devices. The effect of this teaching is to reduce the residual charge in the image areas which becomes more severe when applying color toners onto previously developed color toners, and also to prevent toner spray (or toner spread) during the exposure process. Toner spray is a phenomena caused when the photoconductor carrying the first toner image is recharged to a relatively high charge level and then exposed for the second image development. In areas where the edges of a prior developed image align but do not overlap with the edges of a subsequent image, the toner of the prior image tends to spray or spread along its edges into the subsequently exposed areas which have a relatively lower charge level. By reversing the polarity of the toner as taught in this reference, toner spray is prevented, as the reversed polarity toner is no longer attracted to the exposed areas.
However, when a substantial amount of toner charge at the top of a previously developed toner layer is reversed in polarity during recharge, a different problem of a serious nature develops. Since the prior toner image is now predominantly of an opposite polarity to both the background bare areas and the incoming color toner to be developed thereon, an interaction occurs among these three separate and distinctly charged regions. For example, in a system having a negatively charged photoreceptor using discharged area development (DAD), the negatively charged toner used for development would be reversed in polarity after recharge using the teachings of Matsushita Japanese Patent No. Hei 1-340663. Particularly, the now-positively charged toner layer is then attracted to the negatively charged background areas and the negatively charged toner of the incoming color image. Thus, the positively charged toner of the first image tends to splatter into neighboring bare background regions. This occurrence has been titled the "under color splatter" defect (UCS) and is the cause for the unwanted blending of colors and the spreading of colors from image edges into background areas. The UCS defect is apparent both where the prior image aligns with a subsequent image, and also where the prior image overlaps with the subsequent image. Consequently, color clarity is severely impacted. Furthermore, when a relatively large voltage difference between the first and second charging devices is applied to the photoreceptor surface in order to reverse the polarity of the toner image, a significant amount of stress is applied to the photoreceptor, which may also negatively impact image quality, as well as reduce the life expectancy of the photoreceptor.
Based on the foregoing, a highly reliable and consistent manner of recharging the photoreceptor to a uniform level is needed, whereby the residual voltage on previously toned areas is minimized and the undercolor splatter defect is prevented. Furthermore, a recharging process is needed that does not impair image transfer and that does not unduly stress the photoreceptor. It would also be useful to have a single corona generating device perform two functions in order to reduce the size, weight and cost of a printer. This can be accomplished in a printing machine by having a charging or recharging corona device operate in a first mode for development and imaging purposes and in a second mode for transferring and cleaning purposes.
The following references may be found relevant to the present disclosure.
U.S. Pat. No. 4,791,452 relates to a two-color imaging apparatus wherein a first latent image is formed on a uniformly charged imaging surface and developed with toner particles. The charge retentive surface containing a first developed or toned image, and undeveloped or untoned background areas is then recharged by a scorotron charging device prior to optically exposing the surface to form a second latent electrostatic image thereon. An electrical potential sensor detects the surface potential level of the drum to ensure that a prescribed surface potential level is reached. The recharging step is intended to provide a uniformly charged imaging surface prior to effecting a second exposure.
U.S. Pat. No. 4,761,669 relates to creating two-color images. A first image is formed using the conventional xerographic process. Thus, a charge retentive surface is uniformly charged followed by light exposure to form a latent electrostatic image on the surface. The latent image is then developed. A corona generator device is utilized to erase the latent electrostatic image and increase the net charge of the first developed image to tack it to the surface electrostatically. This patent proposes the use of an erase lamp, if necessary, to help neutralize the first electrostatic image. A second electrostatic image is created using an ion projection device. The ion image is developed using a second developer of a different color.
U.S. Pat. No. 4,833,503 discloses a multi-color printer wherein a a recharging step is employed following the development of a first image. This recharging step, according to the patent is used to enhance uniformity of the photoreceptor potential, i.e. neutralize the potential of the previous image.
U.S. Pat. No. 4,660,059 discloses an ionographic printer. A first ion imaging device forms a first image on the charge retentive surface which is developed using toner particles. The charge pattern forming the developed image is neutralized prior to the formation of a second ion image by a corona generating unit and an erase lamp.
U.S. Pat. No. 5,241,356 discloses a multi-color printer wherein charged area images and discharged area images are created, the former being formed first, followed by an erase step and a recharge step before the latter is formed. An erase lamp is used during the erase step to reduce voltage non-uniformity between toned and untoned areas on a charge retentive surface.
U.S. Pat. No. 5,258,820 discloses a multi-color printer wherein charged area images and discharged area images are created. An erase lamp is used following development of a charged area (CAD), and a pre-recharge corona device is used following development of a discharged area (DAD) and prior to a recharge step, to reduce voltage non-uniformity between toned and untoned images on a charge retentive surface.
The copending application for U.S. patent titled "Method and Apparatus for Reducing Transferred Background Toner", Ser. No. 08/346,708 filed Nov. 30, 1994 by a common assignee as the present application, discloses a corona recharge device for recharging the photoreceptor containing at least one previously developed color image, to a voltage level intermediate to the background areas and the image areas. This intermediate recharge level keeps wrong-charge toner developed in the background areas at a charge level distinct from the toner developed in the image areas, so that the wrong-charge background toner does not transfer to a support substrate with the image.
Another copending application for U.S. patent titled "Split Recharge Method and Apparatus for Color Image Formation", Ser. No. 08/347,617 by a common assignee as the present application, discloses a multi-color imaging apparatus utilizing a recharge step between two image creation steps for recharging a charge retentive surface to a predetermined potential pursuant to forming the second of the two images, a first corona generating device recharges the charge retentive surface to a higher absolute potential than a predetermined potential, and then a second corona generating device recharges the charge retentive surface to the predetermined potential. An electrical charge associated with the first image is substantially neutralized after being recharged by the first and second corona generating device.
U.S. Pat. No. 4,141,648 and 4,432,631 teach a two cycle process electrophotographic copying machine having charging, imaging, developing, transferring, and cleaning facilities, whereas one of the corona devices performs both the final charge leveling and precleaning functions and another corona device performs both the precharging and transferring functions. During the first cycle of the two cycle process, the photoconductor is precharged to a first potential by the combined precharge/transfer corona and then the first potential is augmented to an overcharge by the combined final charge leveling/preclean corona device. Another corona device at the final charge/preclean station reduces and smoothes the overcharge of the photoconductor to the operating potential of the photoconductor and the imaging and developing then occur. In the second cycle, the toned image is transferred to the transfer media using the precharge/transfer corona. Following transfer, the drum is charged by the final charge/preclean corona to a second potential for cleaning.
U.S. Pat. No. 4,835,571 discloses a corona discharging unit which can perform more than one of the functions required in a copying process such as preliminary charging, transferring, discharging and cleaning. This is accomplished by having a shield case with a corona discharge line installed inside of it, a control grid in the vicinity of the open section of the shield case for controlling the polarity and/or output of the corona discharge, a controller connected to the grid for controlling the polarity and/or output of the corona discharge, and a high voltage AC power source connected to the corona discharge line. The corona device has a positive or negative polarity and an AC or DC power source depending upon its desired function in the copying cycle. Various control devices are disclosed.
A number of commercial printers employ the REaD charge/expose/develop/recharge imaging process. For example, the Konica 9028, a multi-pass color printer forms a single color image for each pass. Each such pass utilizes a recharge step following development of each color image. The Panasonic FPC1 machine, like the Konica machine is a multi-pass color device. In addition to a recharge step the FPC1 machine employs an AC corona discharge device prior to recharge.